Display apparatus having temperature compensation function

ABSTRACT

The invention provides a display apparatus having a temperature sensor and a step-up circuit generating a drive voltage of the display apparatus by stepping up a power source voltage. The temperature sensor provides temperature data indicative of the temperature of the display panel with a reduced measurement error without limiting the operation of the step-up circuit. To do so, in view of the fact that the step-up circuit has alternating active state in which the circuit is stepping up a source voltage to output a required step-up voltage and inactive state in which no stepping operation is made, the temperature sensor is adapted to obtain temperature data while the step-up circuit is inactive. The temperature data obtained is supplied to a temperature compensation means for changing the drive condition setting value that define the drive condition of the display panel to compensate for the temperature change.

FIELD OF THE INVENTION

[0001] The invention relates to a display apparatus utilizing, forexample, liquid crystal display (LCD) and organic electroluminescentdisplay (ELD).

BACKGROUND OF THE INVENTION

[0002] Display apparatuses utilizing LCD and ELD have a step-up circuitfor stepping up a given power source voltage to a drive voltage fordriving the display. Expressive power of such display means have beenincreasingly improved in recent years. For example, display apparatusesof compact communications apparatuses such as PDAs and cellular phoneshave a high-resolution, varied gradation ability (ranging frommonochrome to multi-color gradation), multi-color capability (rangingfrom monochrome to full colors). To drive LCD and ELD elements in amulti-color mode, it is necessary to drive them under an optimum drivingcondition. For this reason, it is necessary for a display apparatus tohave means for accurately following the temperature dependent thresholdvoltage and the response time of the LCD and the ELD elements used tocompensate for a change in temperature, correcting the drive conditionsof these elements. In what follows an LCD element will be discussed as atypical example.

[0003] An LCD panel is conventionally equipped with a temperature sensorfor providing digital temperature data of the display panel. The datacan be used to control the display panel to be operated in the optimumcondition (see the Japanese Patent Early Publication No. H5-273941).

[0004] In conventional LCD apparatuses, measurement of the temperatureis performed as it is needed. Therefore, the temperature measurement isperformed independently of the control of the step-up circuit.

[0005] Generally, a temperature sensor is adapted to detect temperatureby detecting a minute change in voltage or current within a circuit.When the step-up circuit is stepping up a power source voltage towards arequired step-up voltage through switching operation of switches (thenthe circuit is referred to as being in active state), it is likely thatthe switching operation generates a voltage noise on, for example, powersupply lines, ground lines, and signal lines involved. Voltage noisealso appears very often when a large drive current is passed to adisplay panel (that is, when the drive current changes rapidly). Thevoltage noise becomes large especially when a display apparatus has astep-up circuit, a temperature sensor, and a drive circuit all formed onthe same semiconductor substrate. Since voltage noise becomes a sourceof error in the temperature measurement, accurate temperaturecompensation of the operating conditions cannot be attained in suchdisplay apparatus. It is possible to reduce the voltage noise byincreasing the capacity of the power supply source or by increasing thethickness of power supply lines and ground lines. However, it is not arealistic solution, since it causes an increase of cost and increasesthe dimensions of the display.

SUMMARY OF THE INVENTION

[0006] It is, therefore, an object of the invention to provide a displayapparatus having a temperature sensor and a step-up circuit forgenerating a drive voltage of the display apparatus, the temperaturesensor capable of detecting the temperature of the display panel withonly a negligible measurement error, without limiting the operation ofthe step-up circuit.

[0007] In accordance with one aspect of the invention, there is provideda display apparatus, comprising:

[0008] a display panel;

[0009] a display memory for storing contents to be displayed on thedisplay panel;

[0010] a step-up circuit for stepping up a power source voltage to apredetermined step-up voltage;

[0011] a drive circuit receiving the step-up voltage as the drivevoltage for driving the display panel based on a drive condition definedby a drive condition setting value to display the contents of thedisplay memory on the display panel;

[0012] a temperature sensor for detecting the temperature of saiddisplay panel and outputting temperature data associated with thetemperature;

[0013] a temperature compensation circuit for changing said drivecondition setting value, when said temperature data has changed in aperiod of time in which the current for driving said display panel donot change appreciably, from the drive condition setting value setbefore said temperature data has changed (said value referred to olddrive condition setting value) to a drive condition setting value inaccord with the temperature after said temperature data has changed(said value referred to as new drive condition setting value); and

[0014] a controller for controlling said display memory, step-upcircuit, drive circuit, temperature sensor, and temperature compensationcircuit.

[0015] In accordance with another aspect of the invention, there isprovided a display apparatus, comprising:

[0016] a display panel;

[0017] a display memory for storing contents to be displayed on thedisplay panel;

[0018] a step-up circuit having alternating active state and inactivestate, outputting a predetermined step-up voltage by stepping up a powersource voltage;

[0019] a drive circuit receiving the step-up voltage as the drivevoltage for driving the display panel based on a drive condition definedby a drive condition setting value to display the contents of thedisplay memory on the display panel;

[0020] a temperature sensor for detecting the temperature of saiddisplay panel and outputting temperature data associated with thetemperature;

[0021] a temperature compensation circuit for changing said drivecondition setting value, when said temperature data has changed whilesaid step-up circuit is in inactive state, from the drive conditionsetting value set before said temperature data has changed (said valuereferred to old drive condition setting value) to a drive conditionsetting value in accord with the temperature after said temperature datahas changed (said value referred to as new drive condition settingvalue); and

[0022] a controller for controlling said display memory, step-upcircuit, drive circuit, temperature sensor, and temperature compensationcircuit.

[0023] In accordance with still another aspect of the invention, thereis provided a display apparatus, comprising:

[0024] a display panel;

[0025] a display memory for storing contents to be displayed on thedisplay panel;

[0026] a charge pump type step-up circuit in continuous step-upoperation stepping up a power source voltage to output a step-upvoltage;

[0027] a drive circuit receiving the step-up voltage as the drivevoltage for driving the display panel based on a drive condition definedby a drive condition setting value to display the contents of thedisplay memory on the display panel;

[0028] a temperature sensor for detecting the temperature of saiddisplay panel and outputting temperature data associated with thetemperature;

[0029] a temperature compensation circuit for changing said drivecondition setting value, when said temperature data has changed in asecond predetermined period of time after a first predetermined periodimmediately after a switch of said charge pump type step-up circuit isswitched ON, from the drive condition setting value set before saidtemperature data has changed (said value referred to old drive conditionsetting value) to a drive condition setting value in accord with thetemperature after said temperature data has changed (said value referredto as new drive condition setting value); and

[0030] a controller for controlling said display memory, step-upcircuit, drive circuit, temperature sensor, and temperature compensationcircuit.

[0031] The temperature sensor is operated only in a predetermined periodwithin an inactive period of said step-up circuit but not in apredetermined initial segment of said inactive period.

[0032] The temperature sensor is continuously operated. In this case,only the temperature data that is obtained in a predetermined periodwithin an inactive period of said step-up circuit but not in apredetermined initial segment of said inactive period is used as thetemperature data for changing said drive condition setting value to saidnew drive condition setting value.

[0033] Alternatively, the temperature sensor may be operated only in asecond predetermined period after a first predetermined periodimmediately after said switch of said charge pump type step-up circuitis switched ON.

[0034] Still alternatively, the temperature sensor may be continuouslyoperated while validating only the temperature data that are detected ina second period after a predetermined period immediately after saidswitch of said charge pump type step-up circuit is switched ON.

[0035] Moreover, the temperature compensation circuit is configured toestablish at least one intermediate drive condition setting valuebetween the old drive condition setting value and the new drivecondition setting value so that the drive circuit undergoes sequentialtransitions from the drive condition defined by the old drive conditionsetting value to the new operating condition defined by the new drivecondition setting value via the intermediate drive condition defined bythe at least one intermediate drive condition setting vale, with apredetermined transition period.

[0036] In the inventive display apparatus, in order to change drivecondition setting data, a temperature compensation circuit uses thetemperature data detected by a temperature sensor in: A period in whichthe step-up circuit is in inactive state (the period referred to asinactive period); a second predetermined period after a firstpredetermined period immediately after the switch of said charge pumptype step-up circuit is switched on; and a period in which the currentdriving the display panel does not change appreciably. Therefore, theinfluence of the voltage noise generated by the stepping up of a sourcevoltage and driving the display panel and appearing on the power supplylines, ground lines, and signal lines can be circumvented. Thus,desirable temperature compensation can be attained.

[0037] Alternatively, the temperature sensor may be operated, as needed,only for a period not influenced by the noise, power consumption by thetemperature measurement can be reduced.

[0038] It is noted that by placing the temperature sensor in constantoperation, and employing the temperature data obtained in a period whichis not influenced by the noise, reliable temperature data can beprovided without any delay.

[0039] The display apparatus may proceed to the new drive conditionsthrough intermediate levels in a finite transition period, avoiding arapid change in drive condition, and a drastic change in brightness forexample of the display screen. The display screen can change so smoothlythat no abrupt change will appear.

BRIEF DESCRIPTION OF THE DRAWINGS

[0040]FIG. 1 is a block diagram representation of an embodiment of asystem according to the invention. FIG. 2 shows a relevant circuithaving a temperature detection function in accordance with a firstembodiment of the invention. FIG. 3 is a circuit diagram of a step-upcircuit. FIG. 4 is a circuit diagram of a temperature sensor. FIG. 5 isa timing diagram of a display apparatus in accordance with an embodimentof the invention. FIG. 6 shows a relevant circuit having a temperaturedetection function in accordance with a second embodiment of theinvention. FIG. 7 is a timing diagram of the display apparatus inaccordance with another embodiment of the invention. FIG. 8 is a blockdiagram representation of a temperature compensation circuit accordingto the invention. FIG. 9 shows an exemplary digital filter. FIG. 10 is atiming diagram of an LCD panel drive.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0041] Preferred embodiments of a display apparatus of the inventionwill now be described with reference to the accompanying drawings.

[0042] Referring to FIG. 1, there is shown a drive 100 for driving anLCD display panel 200 of an LCD apparatus according to the invention.The drive will be referred to as LCD drive. The LCD drive 100 has amodular structure in which different modules or components are formed onthe same semiconductor device. The LCD drive 100 includes such majorcomponents as an external I/F circuit 1, a drive voltage generatingcircuit 2 constituting a drive circuit together with an LCD drivecircuit 3, a display memory 4 comprising a RAM for example, a drivecontroller 5, a setting register 6, a temperature compensation circuit10, a temperature sensor 11, and a step-up circuit 12. The temperaturecompensation circuit 10 is built in the drive controller 5 as shown inFIG. 1. The temperature compensation circuit 10 may be provided withinthe drive voltage generating circuit 2, or as an independent component.Although the step-up circuit 12 is shown as an independent component, itmay be included in the drive voltage generating circuit 2. It should beunderstood that other minor components such a timing circuit are notshown for simplicity, though they are necessary.

[0043] The external I/F circuit 1 interfaces the modular LCD drive 100with external control devices such as an MPU provided outside the LCDdrive 100. The drive voltage generating circuit 2 supplies the LCD drivecircuit 3 with the step-up voltage Vout stepped up by the step-upcircuit 12 and other drive signals. Upon receipt of the drive voltagefrom the drive voltage generating circuit 2 and display data for thedisplay memory 4, the LCD drive circuit 3 drives the LCD panel 200 underthe control of the drive controller 5.

[0044] The setting register 6 stores different sets of settingvalues(parameters) defining different drive conditions of the LCD drive100, which include, for example, levels of output voltage, displaymodes, and frame frequency. The operating conditions of the respectivecomponents (e.g. the drive voltage generating circuit 2, LCD drivecircuit 3, and display memory 4) are determined based on the settingvalues held in the setting register 6.

[0045] The temperature sensor 11 is provided in proximity to the LCDpanel 200 to detect the temperature thereof. The temperature sensor 11is also built in the same semiconductor device as other components. Thesemiconductor device is directly formed on the glass surface of thedisplay panel 200 in the form of so-called COG (chip-on glass)configuration. The temperature sensor 11 starts its operation inresponse to a monitor signal Mt supplied from the drive controller 5,detecting the temperature and providing digital temperature data Tdet.

[0046] The digital temperature data Tdet detected by the temperaturesensor 11 is supplied to the temperature compensation circuit 10. Thetemperature compensation circuit 10 can change the drive condition ofthe LCD panel 200 based on a detected temperature change Tdet, from theold drive condition set before that change to a new drive condition inaccord with the new temperature after the change. This can be done bychanging various operating parameters (e.g. drive voltage, length ofoperating period, and timing), Furthermore, the temperature compensationcircuit 10 sets up at least one (possibly three) set(s) of intermediatedrive conditions between the old drive condition and the new conditions,so that the drive circuit undergoes sequential transitions from the olddrive condition to the new drive condition via the intermediate drivecondition(s) with a predetermined transition period.

[0047] The step-up circuit 12 steps up the source voltage Vdd by meansof a charge pump to generate an output voltage Vout. This step-upvoltage Vout is supplied to the drive voltage generating circuit 2.There are two types of step-up circuits: A first type adapted to stopits operation when the output voltage Vout has reached the target outputvoltage and resume its operation when the output voltage Vout has fallenbelow the target voltage; and a second type adapted to be in constantswitching (ON-OFF) operation. The first step-up circuit has a highswitching frequency (1 MHz, for example), while the second step-upcircuit has a low switching frequency (100 Hz-10 kHz, for example). Inthe example shown herein, the step-up circuit used is of the first type.

[0048] The drive controller 5 is connected to the respective componentsof the LCD drive 100 (e.g. external I/F circuit 1, drive voltagegenerating circuit 2, LCD drive circuit 3, display memory 4, drivecontroller 5, setting register 6, temperature compensation circuit 10,temperature sensor 11, and step-up circuit 12) to control thesecomponents. Moreover, the drive controller 5 manages storage of initialsetting data received from an external MPU in the setting register 6. Inthe example shown herein, the LCD drive 100 has a temperaturecompensation circuit 10, as described above.

[0049] Referring to FIG. 2, there is shown a block diagram illustratingan arrangement of the temperature sensor 11 and the step-up circuit 12,and drive controller 5, which are relevant to detection of temperaturein accordance with the invention. FIGS. 3 and 4 show internal structuresof the step-up circuit 12 and the temperature sensor 11, respectively.

[0050] As shown in FIG. 2, the step-up voltage Vout obtained from thepower source voltage Vdd by the step-up circuit 12 is supplied to thedrive voltage generating circuit 2 and to the inverting input terminal(−) of a comparator 52. A reference voltage circuit 51 adjusts anadjustment circuit in response to an instruction data specifying thedrive voltage supplied from a control circuit 53 to establish apredetermined reference voltage Vref, and supplies the reference voltageVref to the non-inverting input terminal (+) of the comparator 52. Thecomparator 52 compares the step-up voltage Vout with the referencevoltage Vref, and generates an enable signal EN and a busy signal Busywhen the step-up voltage Vout is lower than the reference voltage Vref.The step-up circuit 12 is operable while it is supplied with the enablesignal EN. The control circuit 53 supplies the monitor signal Mt to thetemperature sensor 11 while it is not supplied with s busy signal Busy.While supplied with a monitor signal Mt, the sensor 11 in turn suppliesdigital temperature data Tdet indicative of the temperature at that timeto the temperature compensation circuit 10 built in the control circuit53.

[0051] Referring to FIG. 3, it is seen that the step-up circuit 12 isformed of n stages of charge pump units including a first stage chargepump unit U1 and the n−th stage output charge pump unit Un. The firststage unit U1 is supplied with the power source voltage Vdd (of, forexample, 2V or 3V). Outputted from the output end of the unit Un is thepredetermined step-up voltage Vout (e.g. 10V), which is obtained bystepping up the power source voltage Vdd, via a smoothing circuit Uoconsists of an N-type MOS transistor Qo having a source and a gateconnected together and a capacitor Co connected between the drain of thetransistor Qo and the ground.

[0052] The units U1-Un have the same configuration. Take the unit U1 forexample. It has an N-type MOS transistor Q1 and a capacitor C1. Thesource S of the N-type MOS transistor Q1 is supplied with the powersource voltage Vdd and connected to the gate G of the transistor Q1,thereby forming a so-called diode connection. The substrate of thetransistor is connected to the lowest potential of the circuit or theground potential. The drain D is connected to the sauce S of the N-typeMOS transistor Q2 of the unit U2 in the next stage. The capacitor C1 isconnected at one end thereof to the drain D of the transistor Q1 andconnected at the other end to a clock line (first clock line CLK1).

[0053] It is noted that the capacitors in the odd numbered stages U1,U3, . . . , etc. are connected to a clock line sending a first clockCLK1, while the capacitors in the even numbered units U2, U4, . . . ,etc. are connected to a clock line sending a second clock CLK2.

[0054] The first clock CLK1 and the second clock CLK2 are 2-phase clockswhich have predetermined frequency and the amplitude as the power sourcevoltage Vdd, and have substantially opposite phases each other. Theclock signal CLK1 results from the clock signal clk as it is amplifiedby a first buffer B1. The second clock CLK2 results from the clocksignal clk as it is inverted by an inverting circuit NOT1 and amplifiedby a second buffer B2.

[0055] In the step-up circuit shown in FIG. 3, an oscillator OSCoscillates in response to the enable signal EN, generating the clocksignal clk. This clock signal clk has a high frequency of, for example,1 MHz. When the clock signal clk alternately its voltage between H leveland L level, it is converted, by the first buffer B1, inverting circuitNOT1, and the second buffer B2, into the first clock CLK1 and the secondclock CLK2 having the opposite phases.

[0056] The respective units U1-Un start charge pump operationssimultaneously with the first and the second clocks CLK1 and CLK2. Thus,the power source voltage Vdd is impressed on the respective units insequence, resulting in the step-up voltage Vout at the output terminal.During the operation of the step-up circuit 12, the power source voltageVdd on the power supply line and the voltage on the ground linefluctuate due to the charging of the units, resulting in a noise on thepower supply line and the ground line.

[0057] The temperature sensor 11 as shown in FIG. 4 consists of a bandgap (BG)-type constant voltage circuit 11-1 for generating a constantband gap reference voltage Vbg a temperature detection circuit 11-2which is adapted to output a thermosense-voltage Vt upon receipt of theBG voltage Vbg indicative of ambient temperature, and an A/D converter11-3 which is adapted to change the analog thermosense-voltage Vt into adigital temperature data Tdet. The temperature sensor 11 becomesoperable when it is supplied with the monitor signal Mt, outputting thetemperature data Tdet.

[0058] The BG-type constant voltage circuit 11-1 outputs a constant BGvoltage Vbg, which is not affected by temperature. In the BG-typeconstant voltage circuit 11-1, the BG voltage Vbg is obtained byutilizing two diodes, one having a voltage characteristic having anegative temperature coefficient, and the other one having a positivetemperature coefficient so that any temperature change cancel outbetween the two diodes.

[0059] The temperature detection circuit 11-2 is formed of anoperational amplifier OP1, a resistor R1, and a resistor R2, alltogether forming a non-inverting amplification circuit. The BG voltageVbg is applied to the operational amplifier OP1, which generates at theoutput terminal thereof an amplified constant voltage. The outputterminal of the operational amplifier OP1 is connected to a constantcurrent source S1 via the series diodes D1 and D2. Hence, the outputvoltage of the operational amplifier OP1 minus the voltage drop acrossthe series diodes D1 and D2 is applied to an operational amplifier OP2.The operational amplifier OP2, a resistor R3, and a resistor R4 togetherform another non-inverting amplification circuit. This non-invertingamplification circuit serves as an output circuit. The voltage dropacross the diodes D1 and D2 due to a constant current flowing throughthem changes with temperature, but changes less at higher temperatures.Therefore, the voltage that amounts to the output voltage of theoperational amplifier OP1 minus the voltage drop across the diodes D1and D2 increases with temperature. As a result, a thermosense-voltage Vtthat depends on the temperature is obtained at the output terminal ofthe operational amplifier OP2, as the output voltage of the diodes isapplied to the operational amplifier OP2.

[0060] The A/D conversion circuit 11-3 converts the input analogthermosense-voltage Vt into a digital temperature data Tdet before it isoutputted from the temperature sensor 11.

[0061] The BG-type constant voltage circuit 11-1 and A/D conversioncircuit 11-3 can operate in a stable condition without being influencedappreciably by temperature. The temperature detection circuit 11-2outputs a thermosense-voltage Vt reflecting a temperature change. Itshould be noted, however, that these circuits are energized by the powersource voltage Vdd, and that they are connected to the ground line. As aconsequence, when the power source voltage and/or the ground voltagechange(s), or when these voltage is superposed with a noise, the digitaltemperature data Tdet is likely to contain errors.

[0062] Referring to FIG. 5, operation of the display apparatus of theinvention as shown in FIGS. 1-4 will now be described.

[0063] The comparator 52 compares the step-up voltage Vout of thestep-up circuit 12 with the reference voltage Vref received from thereference voltage generating circuit 51. In order to perform comparisonin a stable manner, the comparator 52 is adapted to exhibit a hysteresischaracteristic about the reference voltage Vref As a result, the step-upvoltage Vout is controlled to be in a range between a maximum voltageVout-u and a minimum voltage Vout-d.

[0064] It is seen in FIG. 5 that the step-up voltage Vout (curve (i))initially has a voltage Vout-u, which is the maximum voltage of thehysteresis. Under this condition, the output of the comparator 52 has alow level L, so that no enable signal EN is outputted (that is, theenable signal EN has L level), and the step-up circuit 12 is in inactivestate.

[0065] As the LCD drive circuit 3 consumes power in driving the displaypanel for example, the step-up voltage Vout goes down. As the step-upvoltage Vout goes down to the minimum voltage Vout-d at time t1 say, theoutput of the comparator 52 will be inverted to high level (H),supplying a high enable signal EN to the step-up circuit 12.

[0066] As shown in FIG. 3, the oscillator OSC of the step-up circuit 12starts oscillation upon receipt of the enable signal EN, causing thestep-up circuit 12 to start its step-up operation. Then, clocked by theclock signal clk of the oscillator OSC, the respective charge-up unitsare charged with the power source voltage Vdd, thereby building up astep-up voltage Vout.

[0067] During this charge-up operation, building up the step-up voltageVout, the voltage Vdd of the power supply line and the voltage of theground line fluctuate as shown by a noise voltage Vnz in curve (iii) ofFIG. 5, due to charging and discharging of the capacitors C1-Cn and Coand ON/OFF switching of the buffers B1 and B2. The noise voltage Vnzgenerated during the step-up operation of the step-up circuit 12 causesthe semiconductor substrate potential to change accordingly. This noisevoltage Vnz continues to remain for a short period of time even afterthe end of step-up operation.

[0068] Referring again to FIG. 5, it is seen that the step-up voltageVout gradually builds up until it reaches the maximum voltage Vout-u attime t2 say. Then, the output of the comparator 52 is inverted to lowlevel L, thereby stopping the enable signal EN (i.e. pulling down thesignal EN to L level). The step-up circuit 12 repeats this operation fora period from t2 to t6 as seen in FIG. 5, maintaining the output voltageVt at substantially the same level as the reference voltage Vref.

[0069] On the other hand, the busy signal Busy (the same as the enablesignal) is supplied to the control circuit 53, causing the controlcircuit 53 to generate the monitor signal Mt, which is supplied to thetemperature sensor 11. The three grounded elements 11-1-11-3 of thetemperature sensor 11 as shown in FIG. 4 are driven by the power sourcevoltage Vdd. As a consequence, the temperature sensor 11 is likely to beaffected by the fluctuation of the supply voltage and the ground voltageas well as by the noise, resulting in errors in the digital temperaturedata Tdet unless the errors are removed.

[0070] It is noted, however, that in the embodiment shown herein, themonitor signal Mt is outputted only in a monitoring period Tm, asdescribed below and shown in FIG. 5 by curve (iv): The monitoring periodTm begin after a period of time a in which the fluctuations ofsemiconductor substrate potential diminished completely following theend of the step-up operation of the step-up circuit 12. In this way,errors that can otherwise take place in the digital temperature dataTdet are eliminated.

[0071] The monitoring period Tm has a sufficient length to detectdigital temperature Tdet several times and to calculate a mean value.The length of an inactive period of the step-up circuit 12 is irregular,like its ON period. Thus, the monitoring period Tm is taken to beshorter than the normally anticipated shortest inactive period.

[0072] Moreover, instead of taking the monitoring period Tm for eachinactive period, it may be taken only once for two or three inactiveperiods. In this instance, the temperature sensor 11 will operate oncein a fixed period that contains multiple inactive periods. When thetemperature of the display panel 200 changes little, compensation can beachieved by less frequent temperature measurements, with reduced powerconsumption.

[0073] The control circuit 53 calculates a mean value of the multipledigital temperature data Tdet received from the temperature sensor 11 inthe monitoring period Tm. When a significant mean value is obtained, thevalue is latched as the temperature data for inactive period. The meanvalue is supplied to the temperature compensation circuit 10.

[0074] When a necessary temperature data are not obtained in themultiple inactive periods, detection of the temperature data for theperiods is abandoned and uses the previously obtained data as the datafor the periods.

[0075] The temperature data collected by the temperature sensor 11 whilethe step-up circuit 12 is in inactive state are used to change drivecondition by the temperature compensation circuit 10. Thus, the voltagenoise generated in the step-up operation by the charge pumps involvedand appearing on the power supply line, the ground line, and the signallines can be prevented from influencing on the temperature data foraccurate temperature compensation.

[0076] Since the temperature sensor 11 is operated only in noise-freeperiods based on the monitor signal Mt generated as needed, powerconsumption by the temperature sensor 11 can be reduced accordingly.

[0077] As another example, only the temperature detection circuit 11-2and/or the A/D conversion circuit 11-3 may be energized by the monitorsignal Mt to suppress power consumption by these power eating elements,while the BG-type constant voltage circuit 11-1 may be put in constantoperation for the reason that it takes some time before it can provide astable output voltage. In this way, both the quick response capabilityand power saving capability of the circuit can be accomplishedsimultaneously. This feature applies to other embodiments.

[0078] As a further example, the whole temperature sensor 11 can be putin continuous operation, as shown in (vi) of FIG. 5. In the case, themonitor signal Mt is not supplied to the temperature sensor, but digitaltemperature data Tdet is outputted from the temperature sensor 11 to thecontrol circuit during a monitoring period. In this configuration, sincethe temperature sensor is always in operation, necessary temperaturedata Tdet can be obtained at any time as needed, though powerconsumption increases accordingly. The feature also applies to otherembodiments.

[0079] Referring to FIG. 6, there is shown s block diagram of atemperature sensor 11, a step-up circuit 12A, and a control circuit 53Aincluded in a drive controller 5 for use in a display apparatus inaccordance with another embodiment of the invention. The LCD drive andthe temperature sensor of this embodiment are the same as the LCD driveshown in FIG. 1 and the temperature sensor as shown in FIG. 4,respectively. FIG. 7 is a timing diagram of the temperature sensor.

[0080] The step-up circuit 12A is a second type of step-up circuithaving an oscillator OSC in continuous operation. Hence, the embodimentdiffers from the one shown in FIG. 3 in that an enable signal EN is notsupplied but instead a clock signal clk is supplied as ON/OFF signalfrom the step-up circuit 12A to the control circuit 53A. As comparedwith the step-up circuit of FIG. 3, capacitances of the capacitors C1-Cnand Co of this embodiment are much larger than those of correspondingcapacitors shown in FIG. 3, and accordingly, the oscillation frequencyof the oscillator OSC is lower by a factor of 10 or 100 (in the rangefrom 100 Hz to about 10 kHz).

[0081] Referring to FIGS. 6 and 7, operation of the display apparatus inaccordance with this embodiment will now be described.

[0082] Since the step-up circuit 12A is in operation all the time, theoutput MOS transistor Qo is turned ON and OFF as shown in FIG. 7 by line(ii). The step-up voltage Vout starts charging the capacitor Co at timet1, when the MOS transistor Qo is turned ON, as shown FIG. 7 by line(i). When the voltage across the capacitor Co rises to the maximumvoltage at time t2, the charging ends. Subsequently, the voltagegradually decreases with time, in accordance with the power consumption.In the embodiment shown herein, the step-up voltage Vout is notcontrolled by any feedback loop. Thus, although the maximum step-upvoltage Vout depends on the load coupled, it does not change rapidly dueto the fact that the capacitor Co has a sufficiently large capacitance.

[0083] As stated above, the power source voltage Vdd and the groundvoltage fluctuate during the period t1-t2 when the output MOS transistorQo is turned ON to raise the step-up voltage Vout. The embodiment shownherein may also avoid errors in the digital temperature data Tdet causedby the voltage fluctuation. To do so, the monitor signal Mt is suppliedto the temperature sensor 11 after a waiting period β subsequent withreference to the switching ON of the temperature sensor 11, allowing forthe fluctuations in the power source voltage Vdd and the groundpotential to decay within the waiting period β as shown in FIG. 7 byline (iv). Thus, the monitor signal Mt is outputted only in the periodsTm in which stable digital temperature data Tdet (line (v) of FIG. 7)are available.

[0084] A multiplicity of digital temperature data Tdet received from thetemperature sensor 11 in the monitoring period Tm are averaged forseveral times, until a significant mean value is obtained. The meanvalue thus obtained is latched, and supplied as the temperature data tothe temperature compensation circuit 10 of the control circuit 53A.

[0085] It is noted that the period from time t3 to t4 also includes avoltage fluctuating period due to the switching of the MOS transistorsin the stages that precedes the output MOS transistor Qo. Therefore, theperiod from t3 to t4 is also avoided to obtain the temperature dataTdet.

[0086] In this way, the temperature data of the temperature sensor 11for use in resetting the drive condition in the temperature compensationcircuit 10, is obtained in a period in which the power supply voltageand the ground potential have become stable, that is, in the monitoringperiod Tm after a predetermined period immediately after the switch ofthe charge pump type step-up circuit is switched ON. Thus, the influenceof the voltage fluctuations and voltage noise generated by the steppingup of a source voltage and appearing on the power supply lines, groundlines, and signal lines can be circumvented. Therefore, adequatetemperature compensation can be attained using accurate temperaturedata.

[0087] In the above description of the invention, validity oftemperature data have been described in connection with the operationalstate of the step-up circuit. However, the invention is not limited tothe embodiments described above. For example, it suffices to invalidatethe temperature data over a period when a large drive currentmomentarily flows through the display panel, because the temperaturedata is then easily affected by the voltage noise caused by a momentarylarge current.

[0088] In the embodiment of the invention shown herein, the polarity ofvoltage applied to the display panel alternates between positive andnegative polarities with a predetermined period. It is noted that afairly large current will flows through the display panel momentarilyduring switching of the polarity as compared with the current inordinary driving operation. A large current also flows through thedisplay panel when a common line of the display panel is switched overfrom one to the other, causing the display elements to be charged ordischarged. By invalidating the temperature data while such a momentarylarge current flows, it is possible to prevent voltage noise fromaffecting the temperature compensation.

[0089] Next, referring to FIGS. 8-10, a scheme of temperaturecompensation will be described, in which, if a temperature change isdetected, intermediate drive conditions are established for thetemperature compensation circuit 10 between an old drive condition setbefore a temperature change to a new (target) drive condition. Thedigital temperature data Tdet outputted from the temperature sensor 11exhibits a finite jump, or changes discontinuously. Such a jump in thetemperature data Tdet results in an abrupt change in brightness of adisplay screen for example. Hence, a sudden noticeable change ofbrightness will appear on the display if the change is large enough. Itis therefore preferable to suppress such sudden change in drivecondition using a temperature compensation circuit as shown in FIGS.8-10, thereby removing unpleasant sensation of viewers.

[0090]FIG. 8 shows a block diagram of the temperature compensationcircuit 10 having multiple temperature compensation families to dealwith different drive conditions. In what follows, however, only onetemperature compensation family will be described, since all thetemperature compensation families are essentially the same in structure.

[0091] A multiplicity of k-bit (k=6 for example) digital temperaturedata Tdet received from the temperature sensor 11 are averaged by anaveraging circuit 13 which functions as a noise filter.

[0092] A temperature-output conversion circuit 14 is adapted to converta temperature data into a corresponding voltage data with a prescribedslope, based on pre-installed slope setting data. In response to inputdigital temperature data, the temperature-output conversion circuit 14outputs a preliminary drive condition setting value in accord with thetemperature data. The preliminary drive condition setting value is ann-bit data (n=8 for example), and has a higher resolution than the inputdigital temperature data. Any temperature-dependent characteristic(including a linear and a non-linear characteristic) of a displayelement can be represented by a multiplicity of slope setting data,since any characteristic curve can be defined in terms of its slope orfirst-order differential. Thus, digital temperature data is convertedinto preliminary drive condition setting value based on the slopesetting data for a particular display panel.

[0093] An addition circuit 15 adjusts the volume of preliminary drivecondition setting value by adding thereto or subtracting therefrom anarbitrary adjustment value, and outputs an m-bit (m=10 for example)drive condition setting value. The temperature-output conversion circuit14 and the addition circuit 15 can be integrated to a temperature outputconversion means.

[0094] A digital filter 16 functions as a means for outputtingtransitional output. (The filter will be referred to astransitional-output means.) The digital filter 16 receives a drivecondition setting value from the addition circuit 15. When the drivecondition setting value has changed, at least one intermediate drivecondition is set up by establishing an intermediate drive conditionsetting value between that value set before the change (referred to asold drive condition setting value) and that value in accord with thetemperature after the change (referred to as new drive condition settingvalue). The drive circuit make sequential transition from the old drivecondition defined by the old drive condition setting value to theintermediate drive condition defined by the intermediate drive conditionsetting value, and further to the new drive condition defined by the newdrive condition setting value, with a period of time constant τ.

[0095] Referring to FIG. 9, there is shown an exemplary digital filter16, which includes a frequency divider 16-1, an up/down counter 16-2,and a comparator 16-3. The frequency divider 16-1 receives a clock of afixed frequency and a time constant τ (of 2 seconds for example) andoutputs a pulse signal P every τ seconds.

[0096] The comparator 16-3 receives a drive condition setting value fromthe addition circuit 15 and an output of the up/down counter 16-2, andcompares the two values. The comparator 16-3 outputs an incrementalsignal UP or a decrease signal DOWN depending on whether the new drivecondition setting value has increased (UP) or decreased (DOWN) ascompared with the old drive condition setting value.

[0097] The up/down counter 16-2 receives a pulse signal P from thefrequency divider 16-1 and either the increment signal UP or decreasesignal DOWN from the comparator 16-3. Every time a pulse signal P isreceived, the up/down counter increment or decrease its drive conditionsetting data by 1 unit (for example, 2 mV) towards said new drivecondition setting value or towards said old drive condition settingvalue, respectively, depending on which of the increment signal UP anddecrease signal DOWN is received, but decreases the drive conditionsetting value by 1 unit (for example, 2 mV) when a decrease signal DOWNis received.

[0098] The output of the temperature sensor 11 is 6-bit, and the outputof the temperature-output conversion circuit 14 is 8-bit. Therefore, ifthe output of the temperature sensor 11 increases (or decreases) by 1unit, the output of the temperature-output conversion circuit 14 isincreased (or decreased) by, for example, 4 units. The number ofincremental/decreasing units depends on the setting of slope. The drivecircuit sequentially undergoes the 4 intermediate drive conditionsassociated with the 4 drive condition setting values, with a transitionperiod of τ.

[0099] The up/down counter 16-2 is initialized to a value associatedwith a standard temperature. As a consequence, the up/down counter 16-2is set to the initial value at the beginning of a startup of theapparatus.

[0100] An D/A converter 17 converts a digital drive condition settingvalue into an analog counterpart. For example, an analog drive conditionsetting value is used as an signal instructing generation of a drivevoltage. When digital drive condition setting value is used as it is,the D/A converter 17 is not necessary.

[0101] Operation of the temperature compensation circuit 10 of the LCDdrive will now be described with additional reference to a timingdiagram of FIG. 10.

[0102] It is assumed here that the temperature of the display panel haschanged and hence the digital temperature data Tdet of the temperaturesensor 11 (6 bit data) has changed accordingly. It is further assumedthat only the lowest significant bit (LSB) of the temperature data Tdethas changed, considering the fact that temperature normally changes onlyslowly.

[0103] In accordance with the change in the LSB of the temperature dataTdet, the 8-bit preliminary drive condition setting value outputted fromthe temperature-output conversion circuit 14 is changed. How much thepreliminary drive condition setting value changes depends on theparticular slope data preset in the temperature-output conversioncircuit 14. Presently, it is assumed that the preliminary drivecondition setting value changes by 4 LSBs. This change is illustrated inFIG. 10 as a change from an old drive condition setting value v1 to anew drive condition setting value v2. It is noted that the operation ofthe addition circuit 15 does not affect this change, since the additioncircuit 15 only adds or subtracts a fixed quantity to or from a valueinput to the addition circuit 15.

[0104] The new drive condition setting value v2 is inputted into thedigital filter 16 as an input signal IN. In the digital filter 16, thecomparator 16-3 compares the input signal IN with the output signal OUT(currently old drive condition setting value v1) of the up/down counter16-2, and generates an incremental signal UP. On the other hand, thefrequency dividers 16-1 generates a pulse signal P every time it countsthe number of clocks that corresponds to the time constant τ.

[0105] As the pulse signal P is inputted at time t1 while theincremental signal UP is outputted, the up/down counter 16-2 incrementsits count by 1, which causes the output drive condition setting value tobe increased by an amount defined by that increment. The amount ofincrease in drive condition setting value that corresponds to anincrement of 1 count is 2 mV for example. Because of this increment ofcount by 1, the level of the output signal OUT is raised. But since theincoming signal IN has a still higher level than the output signal OUT,further incremental signals UP will be generated.

[0106] Under this condition, the count of up/down counter 16-2 isincremented every time a pulse signal P is entered for each timeconstant τ. Accordingly, the output signal OUT gradually rises from theold drive condition setting value v1 to the new drive condition settingvalue v2.

[0107] Eventually, at time t2 in the example shown, the output signalOUT reaches the new drive condition setting value v2. Since at thismoment the input signal IN and the output signal OUT of the comparator16-3 become equal, no incremental signal UP is outputted any more. Thisends the filtering by the digital filter 16.

[0108] In this manner, the temperature-output conversion circuit 14converts a k-bit (6-bit) digital temperature data of the A/D converterinto an n-bit (8-bit) drive condition setting value having a higherresolution. The digital filter 16 takes advantage of this difference inresolution to set up multiple intermediate setting values, so that thedrive circuit can make sequential transitions over the intermediatelevels associated with these intermediate setting values at every timeconstant of τ. Thus, if the input signal IN changes from the old drivecondition setting value v1 to the new drive condition setting value v2in a single step, the output signal OUT undergoes a corresponding changein multiple steps with a period of τ in accord with the intermediatelevels. In this manner, the drive condition of the display apparatus isswitched so gently based on reset drive condition setting value that thedisplay seems to change almost continuously.

[0109] In the above example, an incremental transition from an old drivecondition setting value v1 to the new drive condition setting value v2has been described. It will be apparent that the same procedure can beapplied to the case where a transition is made from an old drivecondition setting value v2 to a lesser new drive condition setting valuev1, provided that the incremental signal UP is replaced by the decreasesignal DOWN.

[0110] It will be also apparent that the temperature compensationcircuit 10 may have a multiplicity of temperature compensation familiesassociated with different drive conditions, as shown in FIG. 8, in whichthe temperature compensation circuit 10 has a separate family thatcomprises a temperature-output conversion circuit 14 a, an additioncircuit 15 a, a digital filter 16 a, and a D/A converter 17 a, sharing acommon temperature sensor 11 and the averaging circuit 13.

What we claim is:
 1. A display apparatus, comprising: a display panel; adisplay memory for storing contents to be displayed on said displaypanel; a step-up circuit for stepping up a power source voltage to apredetermined step-up voltage; a drive circuit receiving said step-upvoltage as the drive voltage for driving said display panel based on adrive condition defined by a drive condition setting value to displaythe contents of said display memory on said display panel; a temperaturesensor for detecting the temperature of said display panel andoutputting temperature data associated with the temperature; atemperature compensation circuit for changing said drive conditionsetting value, when said temperature data has changed in a period oftime in which the current for driving said display panel do not changeappreciably, from the drive condition setting value set before saidtemperature data has changed (said value referred to old drive conditionsetting value) to a drive condition setting value in accord with thetemperature after said temperature data has changed (said value referredto as new drive condition setting value); and a controller forcontrolling said display memory, step-up circuit, drive circuit,temperature sensor, and temperature compensation circuit.
 2. A displayapparatus, comprising: a display panel; a display memory for storingcontents to be displayed on said display panel; a step-up circuit havingalternating active state and inactive state, outputting a predeterminedstep-up voltage by stepping up a power source voltage; a drive circuitreceiving said step-up voltage as the drive voltage for driving saiddisplay panel based on a drive condition defined by a drive conditionsetting value to display the contents of said display memory on saiddisplay panel; a temperature sensor for detecting the temperature ofsaid display panel and outputting temperature data associated with thetemperature; a temperature compensation circuit for changing said drivecondition setting value, when said temperature data has changed whilesaid step-up circuit is in inactive state, from the drive conditionsetting value set before said temperature data has changed (said valuereferred to old drive condition setting value) to a drive conditionsetting value in accord with the temperature after said temperature datahas changed (said value referred to as new drive condition settingvalue); and a controller for controlling said display memory, step-upcircuit, drive circuit, temperature sensor, and temperature compensationcircuit.
 3. The display apparatus according to claim 2, wherein saidtemperature sensor is operated only in a predetermined period within aninactive period of said step-up circuit but not in a predeterminedinitial segment of said inactive period.
 4. The display apparatusaccording to claim 3, wherein the step-up voltage of said step-upcircuit is compared with a predetermined reference voltage; and saidstep-up circuit is controlled to have active and inactive states basedon said comparison such that said step-up voltage remains within apredetermined range with respect to said reference voltage.
 5. Thedisplay apparatus according to claim 4, wherein said step-up circuit isa charge pump type step-up circuit that comprises a multiplicity ofcharge pump units connected in series and an oscillator for generating aclock to be supplied to said charge pump units; and activation andinactivation of said step-up circuit is controlled by the ON-OFFoperation of said oscillator based on said comparison.
 6. The displayapparatus according to claim 3, wherein said temperature sensor isprovided with: a band-gap type constant voltage circuit for generating aconstant band-gap voltage which is little susceptible to temperaturevariation; and a temperature detection circuit including at least onediode for passing therethrough a constant current to generate a voltagedrop across said at least one diode, said temperature detection circuitadapted to generate a thermosense-voltage which varies with the ambienttemperature, using said band gap voltage and said voltage drop acrosssaid diode.
 7. The display apparatus according to claim 3, wherein saidtemperature sensor is periodically operated with a predetermined periodof time that includes a multiplicity of inactive periods of said step-upcircuit.
 8. The display apparatus according to claim 2, wherein saidtemperature sensor is continuously operated; and only the temperaturedata that is obtained in a predetermined period within an inactiveperiod of said step-up circuit but not in a predetermined initialsegment of said inactive period is used as the temperature data forchanging said drive condition setting value to said new drive conditionsetting value.
 9. The display apparatus according to claim 2, whereinsaid temperature compensation circuit establishes at least oneintermediate drive condition setting value between said old drivecondition setting value and said new drive condition setting value sothat said drive circuit undergoes sequential transitions from the drivecondition defined by said old drive condition setting value to the newoperating condition defined by said new drive condition setting valuevia the intermediate drive condition defined by said at least oneintermediate drive condition setting vale, with a predeterminedtransition period.
 10. The display apparatus according to claim 9,further comprising a multiplicity of said temperature compensationcircuits in accordance with a multiplicity of different driveconditions, wherein said multiplicity of temperature compensationcircuits receive the same temperature data from a common temperaturesensor.
 11. The display apparatus according to claim 9, wherein saidtemperature compensation circuit includes: a temperature-outputconversion circuit for converting k-bit digital temperature data intom-bit (m>k) drive condition setting value based on drive conditionsetting information that contains slope data; and a transitional outputcircuit for causing, upon receipt of drive condition setting value fromsaid temperature-output conversion circuit based on parameters thatinclude a time constant, said drive circuit to undergo sequentialtransitions from the drive condition defined by said old drive conditionsetting value to the new operating condition defined by said new drivecondition setting value via the intermediate drive condition defined bysaid at least one intermediate drive condition setting vale, with atransition period.
 12. The display apparatus according to claim 11,wherein said transitional output circuit includes: a frequency dividerreceiving a clock of fixed frequency and said time constant, foroutputting a pulse signal at every time constant; a comparator,receiving driving condition setting value from said temperature-outputconversion circuit, for outputting either an incremental signal or adecrease signal depending on whether said new drive condition settingvalue is larger or smaller than said old drive condition setting value;and an up/down counter included in said transitional output circuit andreceiving a pulse signal from said frequency divider and either saidincrement signal or decrease signal from said comparator, said up/downcounter adapted to increment or decrease its drive condition settingdata by 1 unit, depending on which of the increment signal and decreasesignal is received, every time said pulse signal is received.
 13. Adisplay apparatus, comprising: a display panel; a display memory forstoring contents to be displayed on said display panel; a charge pumptype step-up circuit in continuous step-up operation stepping up a powersource voltage to output a step-up voltage; a drive circuit receivingsaid step-up voltage as the drive voltage for driving said display panelbased on a drive condition defined by a drive condition setting value todisplay the contents of said display memory on said display panel; atemperature sensor for detecting the temperature of said display paneland outputting temperature data associated with the temperature; atemperature compensation circuit for changing said drive conditionsetting value, when said temperature data has changed in a secondpredetermined period of time after a first predetermined periodimmediately after a switch of said charge pump type step-up circuit isswitched ON, from the drive condition setting value set before saidtemperature data has changed (said value referred to old drive conditionsetting value) to a drive condition setting value in accord with thetemperature after said temperature data has changed (said value referredto as new drive condition setting value); and a controller forcontrolling said display memory, step-up circuit, drive circuit,temperature sensor, and temperature compensation circuit.
 14. Thedisplay apparatus according to claim 13, wherein said temperature sensoris operated only in a second predetermined period after a firstpredetermined period immediately after said switch of said charge pumptype step-up circuit is switched ON.
 15. The display apparatus accordingto claim 13, wherein said temperature sensor is further operated, with afixed period of time that includes a multiplicity of switching operationof said switch.
 16. The display apparatus according to claim 13, whereinsaid temperature sensor is continuously operated; only the temperaturedata that are detected in a second period after a predetermined periodimmediately after said switch of said charge pump type step-up circuitis switched ON is used to change said drive condition setting value. 17.The display apparatus according to claim 13, wherein said temperaturecompensation circuit establishes at least one intermediate drivecondition setting value between said old drive condition setting valueand said new drive condition setting value so that said drive circuitundergoes sequential transitions from the drive condition defined bysaid old drive condition setting value to the new operating conditiondefined by said new drive condition setting value via the intermediatedrive condition defined by said at least one intermediate drivecondition setting vale, with a predetermined transition period.
 18. Thedisplay apparatus according to claim 17, further comprising amultiplicity of said temperature compensation circuits in accordancewith a multiplicity of different drive conditions, wherein saidmultiplicity of temperature compensation circuits receive the sametemperature data from a common temperature sensor.